In recent years, research and development for light emitting elements using organic compounds having light emitting properties, have been actively pursued. A basic structure for these light emitting elements is that of interposing a layer including an organic compound having a light emitting property between a pair of electrodes. By applying voltage to such an element, electrons and holes are injected from the pair of electrodes into the layer including an organic compound having a light emitting property, and current flows. Then, by those carriers (electrons and holes) recombining, the organic compound having a light emitting property forms an excited state, and light is emitted when the excited state returns to a ground state. Due to such mechanism, such a light emitting element is called a current excitation light emitting element.
Note that as types of excitation states which an organic compound forms, a singlet excited state and a triplet excited state are possible. Light emission from a singlet excited state is called fluorescence, and light emission from a triplet excited state is called phosphorescence.
Since such a light emitting element is usually formed as an approximately submicron-thin film, there is a great advantage that it can be manufactured to be thin and light weight. Furthermore, since the time it takes from carrier injection to light emission is about microseconds or less, another feature is that the response speed is extremely fast. It is thought that as an element for use in flat panel displays, these features are suitable.
Also, since these light emitting elements are formed in film forms, planar light emission can be easily obtained by forming a large-area element. Since this is a trait that is difficult to obtain by a point light source typified by incandescent lamps and LEDs, or by a line light source typified by fluorescent lights, utility value as a surface light source that can be applied to illumination is also high.
In this manner, there are hopes for the current excitation light emitting element using an organic compound material having a light emitting property to be applied to light emitting devices and illuminations; however, many challenges still remain. Reduction in power consumption is one of those challenges. In order to reduce power consumption, it is important to reduce the driving voltage of the light emitting element. Further, since the emission intensity of the current excitation light emitting element is determined by the amount of electrical current flowing, in order to reduce the driving voltage, it is necessary to feed many currents at low voltage.
Previously, as a method for reducing driving voltage, an approach of providing a buffer layer between an electrode and the layer including an organic compound having a light emitting property, has been attempted. For example, it is known that driving voltage can be reduced by providing a buffer layer that is formed of polyaniline (PAni) doped with camphorsulfonic acid, between indium tin oxide (ITO: indium tin oxide) and a light emitting layer (for example, refer to Non-Patent Document 1: Y. Yang, et al. Applied Physics Letters, Vol. 64 (10), 1245-1247 (1994)). It is explained that this is because of the excellent carrier injecting property of PAni to the light emitting layer. Note that in the Non-Patent Document 1, PAni that is the buffer layer is also considered to be a part of an electrode.
However, as described in the Non-Patent Document 1, PAni has a problem that transmittance becomes poor when a film thickness becomes thick. Specifically, it is reported that at a film thickness of about 250 nm, the transmittance is less than 70%. In other words, since the problem is with the transparency of the material itself that is used for the buffer layer, light that is generated within an element cannot be taken out efficiently.
Also, according to a Patent Document 1: Japanese Patent Laid-Open No. 2003-272860, an approach of serially connecting light emitting elements (called a light emitting unit in the Patent Document 1) to improve the luminance per a certain current density, in other words, current efficiency, as been attempted. As for the Patent Document 1, for a connecting portion of when light emitting elements are serially connected, a mixed layer of an organic compound and a metal oxide (specifically, vanadium oxide and rhenium oxide) is applied, and it is considered that this layer can inject holes and electrons to a light emitting unit.
However, as apparent by looking at an embodiment, for the mixed layer of an organic compound and a metal oxide that is disclosed in the Patent Document 1, a high absorption peak is observed not only in the infrared region but also in the visible light region (around 500 nm), a problem in transparency occurs also. Therefore, as expected, light that is generated within an element cannot be taken out efficiently, and the light emission efficiency of the element is degraded.